Russia says a deadly March sarin attack in an Aleppo suburb was carried out by Syrian rebels, not forces loyal to President Bashar Assad, and it has delivered a 100-page report laying out its evidence to the United Nations.

A statement posted on the Russian Foreign Ministry website late Wednesday said the report included detailed scientific analysis of samples that Russian technicians collected at the site of the alleged attack, Khan al Asal in northern Syria. The attack killed 26 people.

A U.N. spokesman, Farhan Haq, confirmed that Russia delivered the report in July.

The report itself was not released. But the statement drew a pointed comparison between what it said was the scientific detail of the report and the far shorter intelligence summaries that the United States, Britain and France have released to justify their assertion that the Syrian government launched chemical weapons against Damascus suburbs on Aug. 21. The longest of those summaries, by the French, ran nine pages. Each relies primarily on circumstantial evidence to make its case, and they disagree with one another on some details, including the number of people who died in the attack.

The Russian statement warned the United States and its allies not to conduct a military strike against Syria until the United Nations had completed a similarly detailed scientific study into the Aug. 21 attack. It charged that what it called the current “hysteria” about a possible military strike in the West was similar to the false claims and poor intelligence that preceded the U.S. invasion of Iraq in 2003.

Russia said its investigation of the March 19 incident was conducted under strict protocols established by the Organization for the Prohibition of Chemical Weapons, the international agency that governs adherence to treaties prohibiting the use of chemical weapons. It said samples that Russian technicians had collected had been sent to OPCW-certified laboratories in Russia.

“The Russian report is specific,” the ministry statement said. “It is a scientific and technical document.”

The Russian statement said Russian officials had broken the Organization for the Prohibition of Chemical Weapons’ code of silence on such probes only because Western nations appear to be “preparing the ground for military action” in retaliation for the Aug. 21 incident.

A U.N. team spent four days late last month investigating the Aug. 21 incident. The samples it collected from the site and alleged victims of the attack are currently being examined at the chemical weapons organization’s labs in Europe. U.N. Secretary-General Ban Ki-moon has urged the United States to delay any strike until after the results of that investigation are known. But U.S. officials have dismissed the U.N. probe, saying it won’t tell them anything they don’t already know.

White House spokeswoman Caitlin Hayden said U.S. officials were unmoved by the Russian report and held the Assad government responsible for both the Khan al Asal attack in March and the Aug. 21 attack outside Damascus.

“We have studied the Russian report but have found no reason to change our assessment,” she said.

Independent chemical weapons experts contacted by McClatchy said they were not familiar with the report and had not read the Russian statement, which was posted as Secretary of State John Kerry was appearing before the House Foreign Affairs Committee to make the Obama administration’s case for a retaliatory strike on Syria as punishment for the August attack. But they were cautious about the details made public in the Russian statement.

Richard Guthrie, formerly project leader of the Chemical and Biological Warfare Project of the Stockholm International Peace Research Institute, said the Russian statement on the makeup of the sarin found at Khan al Asal, which the Russians indicated was not military grade, might reflect only that “there are a lot of different ways to make sarin.”

He added: “The messy mix described by the Russians might also be the result of an old sarin stock being used. Sarin degrades (the molecules break up) over time and this would explain a dirty mix.”

He also said there could be doubts about the Russian conclusion that the rockets that delivered the sarin in the March 19 incident were not likely to have come from Syrian military stocks because of their use of RDX, an explosive that is also known as hexogen and T4.

“Militaries don’t tend to use it because it’s too expensive,” Guthrie said. He added in a later email, however, that it’s not inconceivable that the Syrian military would use RDX “if the government side was developing a semi-improvised short-range rocket” and “if there happened to be a stock available.”

“While I would agree that it would be unlikely for a traditional, well-planned short-range rocket development program to use RDX in that role, it is not beyond the realms of possibility that, as the Syrian government did not seem to have an earlier short-range rocket program, it may have been developing rockets with some haste and so using materials that are at hand,” he wrote.

Jean Pascal Zanders, a leading expert on chemical weapons who until recently was a senior research fellow at the European Union’s Institute for Security Studies, questioned a Russian assertion that the sarin mix appeared to be a Western World War II vintage.

“The Western Allies were not aware of the nerve agents until after the occupation of Germany,” he wrote in an email. “The USA, for example, struggled with the sarin (despite having some of the German scientists) until the 1950s, when the CW program expanded considerably.”

The Russian Foreign Ministry posted the statement shortly after Russian President Vladimir Putin had asked a Russian interviewer what the American reaction would be if evidence showed that Syrian rebels, not the Assad regime, had been behind a chemical weapons attack.

The report dealt with an incident that occurred March 19 in Khan al Asal, outside Aleppo, in which 26 people died and 86 were sickened. It was that incident that the U.N. team now probing the Aug. 21 attack was originally assigned to investigate, and the Russian statement noted that the investigation had been sidetracked by the sudden focus on the later incident.

Haq, the U.N. spokesman, acknowledged that the most recent attack “has pushed the investigation of the Aleppo incident to the back burner for now.” But he said that “the inspectors will get back to it as soon as is possible.”

The statement’s summary of the report said that neither the munitions nor the poison gas in the Khan al Asal attack appeared to fit what is possessed by the Syrian government. The statement said Russian investigators studied the site, sent the materials they found to study to the Russian laboratories of the Organization for the Prohibition of Chemical Weapons, and followed agreed-upon United Nations investigation standards.

According to the statement, the report said the shell “was not regular Syrian army ammunition but was an artisan-type similar to unguided rocket projectiles produced in the north of Syria by the so-called gang ‘Bashair An-Nasr.’”

The Russian analysis found soil and shell samples contained a sarin gas “not synthesized in an industrial environment,” the statement said. The report said the chemical mix did not appear to be a modern version of the deadly agent but was closer to those “used by Western states for producing chemical weapons during World War II.”

The statement said the Organization for the Prohibition of Chemical Weapons team had examined Syrian soldiers injured in the March attack and said that no reaction to the more recent alleged chemical account should be considered without also considering that the rebels, too, have used chemical weapons.

“It is obvious that any objective investigation of the incident on Aug. 21 in East Ghouta is impossible without considering the circumstances of the March attack,” the statement said. Ghouta is the area near Damascus where the Aug. 21 attack took place.

(Lesley Clark contributed to this report from St. Petersburg, Russia.)

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